Conventional automatic transmissions include transmission fluid, which is maintained within operational volume limits, and a pump, which is driven by rotary motion from a vehicle's engine. The pump circulates the fluid through the transmission and the vehicle's heat exchanger. Among other things, this fluid provides lubrication for the transmission. However, the fluid must be circulating so that all components which need lubrication receive it.
When a vehicle's engine is not operating, the pump does not operate, and the fluid does not circulate. If components within the transmission continue to move against one another while the engine is not operating, adequate lubrication may not reach various critical transmission components. Unless special precautions are taken, this happens any time a vehicle's wheels rotate due to forces other than those supplied by the vehicle's engine. Thus, most automotive manufacturer's caution against towing, pushing, or coasting a vehicle for lengthy distances or at high speeds.
These characteristics of automatic transmissions present a long-recognized problem to persons who wish to tow vehicles having such automatic transmissions. While a short distance, such as to a nearby repair shop, may be traversed at slow speeds without great inconvenience, the towing prohibition imposes a great burden when a vehicle is to be towed a long distance. Conventionally, either of two types of solutions have been applied to this problem.
The first type of solution attempts to prevent a transmission from operating while a vehicle is being towed. One example of this type of solution calls for the use of a trailer. At least the vehicle's wheels to which the transmission couples are placed and held stationary on the trailer while the trailer's wheels rotate to tow the vehicle. This solution is particularly undesirable because the trailer represents a large expense, the vehicle is typically difficult to stow upon the trailer, the trailer takes up valuable space when it is not being used, and the trailer adds a significant weight penalty for a towing vehicle when it is being used.
Another example of preventing a transmission from operating disconnects a drive shaft so that the transmission does not couple to the vehicle's wheels. While this solution prevents a transmission from operating when the vehicle is being towed, it is undesirably inconvenient. Someone must perform work under the vehicle in order first to disconnect the drive shaft for towing and second to reconnect the drive shaft for operating the vehicle under its own power. This is a time consuming and dirty task which requires the availability of tools and a degree of mechanical aptitude. Consequently, this solution is too cumbersome and inflexible whenever the vehicle is towed more than rarely or by persons who are not comfortable performing mechanical tasks.
To address problems specific to disconnecting a drive shaft, an alternate solution incorporates a clutching mechanism in the vehicle's drive shaft. The clutching mechanism is typically operated from a control knob located in the driver's compartment. While the clutching mechanism makes decoupling the transmission from the vehicle's wheels much more convenient, it subjects the drive shaft to becoming unbalanced, and it adds a weak link to the vehicle's power train. Consequently the vehicle's reliability suffers.
In addition, the solutions which decouple a vehicle's drive shaft from its transmission simply cannot be applied to vehicles which do not have drive shafts. Since more and more vehicles are being manufactured as front wheel drive vehicles, which have transaxles rather than drive shafts, these solutions are available only for fewer and fewer vehicles. However, the second of the two types of solutions may be applied to either rear wheel drive or front wheel drive vehicles. This second type of solution continues to circulate transmission fluid when the vehicle is being towed, even though the transmission pump is not operational.
In one example of the second type of solution, an external pump is mounted in the vehicle and coupled to the vehicle's transmission through valves. This system includes an electrical controller and is powered from a towing vehicle. The controller operates the pump and valve and monitors the circulation of fluid for sufficient pressure to insure transmission lubrication. While this solution solves many problems related to the above-discussed techniques, which strive to decouple a drive shaft, it suffers from its own problems. For example, this solution is unduly complicated and expensive. It generally requires the services of a skilled mechanic for installation, and it includes several components which are prone to failure. In particular, the use of an external pump and of a wiring harness and control system which traverses the distance between the towed vehicle and a towing vehicle are highly undesirable.